Control system for a d.c. motor

ABSTRACT

A d.c. motor control system, intended primarily for use in battery powered vehicles, includes a control switch (RL2a, RL2b) for varying the connections of the motor field winding (24) to determine the mode of operation of the motor and speed interlock means for preventing said control switch being operated to change the motor mode while the motor is actually running. Instead of using a mechanical speed transducer the speed interlock means includes means for supplying current to the motor field winding (24) when current would not otherwise be flowing therethrough and means (BR1, T 1 , P 16  etc.) connected to detect the voltage across the armature which occurs in these circumstances if the motor is in motion.

This is a continuation of application Ser. No. 224,224 filed as PCTGB78/00046, Dec. 4, 1978, published as WO79/00355, Jun. 28, 1979,§102(e) date July 23, 1979 now adbondoned.

TECHNICAL FIELD

This invention relates to a control system for a d.c. motor.

BACKGROUND ART

Control systems have previously been proposed for electric vehiclemotors in which there are interlocks between various driver operablecontrols and control devices in the control system to prevent damage tothe system. For example, one such interlock may be provided to prevent acontrol device in the form of a contactor controlling the connection ofthe motor for reverse and forward motoring from being operated exceptwhen the vehicle is at rest. Such an interlock, which will be referredto hereinafter as "a speed interlock", necessitates the provision ofmeans determining whether or not the vehicle is in motion.

In the previous proposals the control system has included a speedtransducer which is used both for providing logic signals for use in thespeed interlock functions and for providing analog signals for thecontrol system. The speed transducer was a mechanical device driven bythe traction motor and producing a pulse train, the frequency of whichwas proportional to speed. Such a transducer added considerably to thecost and complication of the control system and, being a mechanicaldevice, required maintenance.

It is an object of the invention to provide a control system for anelectric vehicle traction motor incorporating at least one speedinterlock function but no mechanical speed transducer.

DISCLOSURE OF INVENTION

In accordance with the invention there is provided a control system fora d.c. motor comprising control means for varying the connections of themotor armature winding and/or field winding so as to enable the motor tooperate in a plurality of different modes, and speed interlock means forpreventing operation of said control means to change the motorconnections from at least one mode to at least one other mode whilst themotor is running characterised in that said interlock means is sensitiveto the voltage across the armature winding of the motor and means areprovided for supplying a field current to the field winding when saidinterlock means is required to be operative.

Thus, when a change over from forward drive to reverse drive isdemanded, for example, the speed interlock means will be required to beoperative at a time when the field and armature currents would otherwisebe zero. When it is required for the speed interlock means to beoperative a current pulse is applied to the field winding. If the motoris at rest the voltage across the armature winding will be zero. If, onthe other hand, the motor is running, a voltage signal will be generatedby the motor which signal is detected and used to prevent thechange-over.

Alternatively it can be arranged for the field current to have apredetermined minimum level below which it is never allowed to fall.

BRIEF DESCRIPTION OF DRAWINGS

An example of the invention as applied to an electrical vehicle d.c.traction motor control system is shown in the accompanying drawings inwhich,

FIG. 1 is a block diagram of the armature and field winding currentcontrols,

FIG. 2 is a block diagram of a logic circuit associated with the circuitof FIG. 1,

FIG. 3 is an electrical circuit diagram of the armature current controlportion of FIG. 1,

FIG. 4 is an electrical circuit diagram of an armature chopper circuitand current signal generator circuit forming part of FIG. 1,

FIG. 5 is an electrical circuit diagram of a field current controlcircuit portion of FIG. 1,

FIG. 6 is an electrical circuit diagram of a field chopper circuitforming part of FIG. 1, and

FIGS. 7a, 7b and 7c together make up the electrical circuit diagram ofthe logic circuit of FIG. 2.

BEST MODE OF CARRYING OUT THE INVENTION

Referring firstly to FIG. 1, the armature current control makes use ofan armature chopper circuit 10 which is shown in detail in FIG. 4. Threethyristor drive circuits 11, 12, 13 control the chopper circuit 10,these drive circuits deriving their input from a Schmitt bistablecircuit 14. This Schmitt bistable circuit 14 receives its input from adifference amplifier 15 which receives at one input terminal a signalrepresenting the instantaneous armature current demand and at its otherinput terminal a current feedback signal from a current signalgenerating circuit 16. The current demand signal is generated byselecting the larger of two voltages on the sliders of twopotentiometers 17, 18 operated respectively by accelerator and brakepedals controlled by the driver of the vehicle in which the system isinstalled. Such selection is carried out by a comparator circuit 19. Thepotentiometers 17, 18 are connected between respective output terminalsof a demand shaping circuit 20 intended to reduce the maximum possibledemand signals with increasing vehicle speed. In fact such shaping iscarried out without the use of a mechanical speed transducer, signalsalready present in the system being used instead. This arrangement isdescribed in detail in copending U.S. application number 224,222 of evendate a common assignment herewith to which reference may be had for afull description of the circuit 20. A demand detector circuit 21 is alsoconnected to the sliders of the potentiometers 17, 18 and provides acontrol signal to the field current control as well as an input to thelogic circuit of FIG. 2.

The field current control includes a field chopper circuit 22 whichsupplies current via a forcing and reversing relay network 23 to themotor field winding 24. The field chopper circuit 22 is connected to aSchmitt bistable circuit 25 via an opto-isolator 26. The Schmittbistable circuit receives a linear signal from a field currentcomparator 27 which receives one input from a field current demandsignal generator 28 and another from a field current signal generator16a. The field current demand signal generator 28 has inputs from theoutput of the comparator 19, from the demand detector 21, from variouspoints in the logic circuit of FIG. 2 and also from a field weakeningdifference amplifier 29. This latter amplifier 29 receives inputs from astart up circuit 30 and from a further amplifier A5 which is operativeduring braking to compare the output of amplifier 15 with a fixedreference value.

Before turning to the detailed circuit diagrams the block logic diagramof FIG. 2 will be briefly explained. Basically the function of the logiccircuit is to control relays RL1 and RL2 in the field current controland a contactor RL3 in the armature current control, and also to controlthe sequence of events when the system is switched on and off, so that aproperly controlled power up and power off sequence occurs. As ignitionswitch circuit 40 controls the supply of power to a power relay 41 whichcontrols the supply of current to regulators 42 for supplying regulatedvoltages to the electronic control and logic circuits and also to a mainisolator 48 (shown in FIG. 4). There are also various interlockscontrolling the energising and deenergising of the power relay 41, suchinterlocks ensuring that the system cannot be brought into operationwhen the vehicle battery is on charge, when the 12V auxiliary battery isdischarged, when the vehicle is moving or when there is an armaturecurrent demand signal present.

A block 43 in FIG. 2 represents a logic circuit (controlling the relayRL1) which is effective during change over between the various motoroperating modes i.e. forward motoring, reverse motoring and braking, andserves to ensure that the field current is rapidly reduced to zero,before the newly selected mode is brought into operation. A logiccircuit 44 controls the relay RL2 and the contactor RL3 and alsoprovides inputs to phase check gates 45 which are responsible forbringing the field forcing logic 43 into operation. These gates detectthe condition which occurs when the driver has selected a new mode ofoperation but the various conditions required for change-over have notyet been met.

For providing a speed logic input to the interlock circuit 41a and thelogic circuit 44 there is an armature voltage detector 46 (shown in FIG.7c) which is used instead of a speed transducer.

Finally FIG. 2 shows a demand clamping circuit 47 which receives inputsfrom the field force logic 43, the logic circuit 44 and from "proving"contacts on the contactor RL3, to control demand clamping signalsapplied to the comparator circuit 19, and the field current demandsignal generator 28, during changes in mode of operation to preventdemand signals being generated at these times.

Turning now to FIG. 3, the armature current control circuit includes thedemand shaping circuit 20 which applies a maximum voltage appropriate tothe existing vehicle speed to the non-earthed ends of the potentiometers17 and 18. The comparator circuit 19 includes an operational amplifierA1 (which is a current differencing type operational amplifier such as1/4 National semiconductors type LM 3900). The sliders of the twopotentiometers 17, 18 are connected by respective resistors R₁, R₂ tothe inverting and noninverting input terminals of amplifier A₁.Hysteresis is provided by a feedback resistor R₃ to the non-invertinginput terminal. The output terminal of amplifier A₁ is connected to aterminal marked MTR/BK which is connected to the logic circuit 44 and isat high voltage when the voltage on the slider of the brakepotentiometer is higher than that on the slider of the acceleratorpotentiometer.

The demand detector 21 of FIG. 1 is represented by an npn transistor N₁with its base connected to the common point of two resistors R₄ and R₅connected in series between the sliders of the potentiometers 17 and 18.The emitter of the transistor N₁ is connected to the earth rail of thesupply and it is biased so as to be just non-conductive when there is azero voltage at the sliders of both potentiometers 17, 18. The biascircuit for the transistor N₁ consists of a pair of resistors R₆, R₇ inseries between the base of the transistor N₁ and the +8v supply rail, aresister R₈ connecting the base of the transistor N₁ to the earth railand a diode D₁ with its anode connected to the common point of resistorsR₆, R₇ and its cathode connected to the earth rail. A capacitor C₁ isconnected between the base of the transistor N₁ and the earth rail. Avery small voltage on the slider of either potentiometer 17 and 18 willsuffice to turn on transistor N₁ which has its collector connected tothe +8v rail by two resistors R₉, R₁₀ in series. The common point ofthese resistors R₉, R₁₀ is connected to the base of a pnp transistor P₁which has its emitter connected to the +8v rail and its collectorconnected to a terminal marked D>O which is connected to interlockcircuit 41. A diode D₂ connects the collector of the transistor N₁ to aterminal marked b (see FIG. 5).

The motor/brake comparator circuit 19 also includes an arrangementwhereby only the larger of the two voltages at the sliders of the twopotentiometers 17, 18 is passed on to the amplifier 15. This arrangementincludes a pair of npn transistors N₂, N₃ with their bases connected byrespective resistors R₁₁, R₁₂ to the sliders of the potentiometers 18and 17 respectively. The emitters of these transistors N₂, N₃ areconnected together and a common resistor R₁₃ connects them to earth. Apair of capacitors C₂, C₃ connect the bases of the transistors N₂, N₃ tothe earth rail and the collectors of these transistors are connectedtogether and via a common resistor R₁₄ to the cathode of a diode D₃, theanode of which is connected to the +8v rail.

It will be appreciated that only the transistor N₂ or N₃ which has itsbase at the higher voltage will conduct at any given time and thattransistor will then act as an emitter follower, so that the voltage onthe resistor R₁₃ will be just one V_(be) below the voltage at the sliderof the appropriate potentiometer. The resistors R₁₁ and R₁₂ and thecapacitors C₂, C₃ act to limit the rate of change of the output voltageof the circuit, each R-C circuit having a time constant of some 70 mS.These also act as noise filters.

Two terminals M and B of the demand clamp circuit 47 (see also FIG. 7b)are connected directly to the bases of the transistors N₃ and N₂ so thatwhen the signals at these terminals are both low, the transistors areturned off. There are two output terminals c and e shown in FIG. 3.Terminal c is connected to the emitters of the transistors N₂ and N₃ andalso to the field current control (FIG. 5). Terminal e is connected tothe collector of a pnp transistor P₂ which has its base connected to thecollectors of the transistors N₂ and N₃, its collector connected to the+8v rail by a resistor R₁₆ and its emitter connected to the same rail bya resistor R₁₇. Terminal e provides an input to the field current demandsignal generator 28 (see also FIG. 5) and acts as a current sourceproviding a current with a minimum level determined by R₁₇ andincreasing linearly with the voltage at the emitter of the transistorsN₂ and N₃.

The amplifier 15 which is another integrated circuit currentdifferencing operational amplifier (e.g. 1/4 LM 3900) is connected tooperate as a linear difference amplifier. To this end the non-invertinginput terminal of the amplifier 15 is connected by a resistor R₁₈ to theoutput of an R.C. filter circuit R₁₉, C₄ across the resistor R₁₃, and isalso connected by a resistor R₂₀ and a variable resistor R₂₁ in to the+8v rail (to provide bias current). The inverting input terminal ofamplifier 15 is connected by a bias resistor R₂₂ to the +8v rail and bya resistor R₂₃ to a terminal marked I_(sig) from the current signalgenerator circuit 16 (see also FIG. 4). The output terminal of theamplifier 15 is connected by a capacitor C₅ to the earth rail and alsoto the anode of a diode D₄. The cathode of the diode D₄ is connected bya feedback resistor R₂₄ to the inverting input terminal of the amplifier15.

The amplifier 15 produces an output signal which is linearly related tothe error between the demanded armature current and the actual armaturecurrent as measured by circuit 16.

This error signal is applied via a resistor R₂₅ to the inverting inputterminal of an operational amplifier A₂ on which the Schmitt bistablecircuit 14 is based. The noninverting input terminal of the amplifier A₂is connected to the +8 v rail by a bias resistor R₂₆ and normal feedbackis provided via a resistor R₂₇ between the output terminal of theamplifier A₂ and its non-inverting input terminal to provide hysteresis(i.e. to set the switching threshold voltages above and below thatdetermined by the biasing resistor R₂₆). The Schmitt bistable circuithas outputs to the three thyristor drive circuits 11, 12 and 13. Theoutput to the thyristor drive circuit 11 is taken from the common pointof the two resistors R₂₈ and R₂₉ in series between the earth rail andone side of a capacitor C₆. The other side of capacitor C₆ is connectedto the collector of a pnp transistor P₃ which has its emitter connectedto the +8v rail, its collector connected by a load resistor R₃₀ to theearth rail and its base connected to the common point of two resistorsR₃₁, R₃₂ connected in series between the +8v rail and the outputterminal of the amplifier A₂. The transistor P₃ turns on when the outputof the amplifier A₂ is low so that there is a positive going outputpulse delivered to drive circuit 11 when the output of amplifier A2 goeslow. This occurs when the actual armature current falls below the demandcurrent by more than the margin established by the hysteresis of theSchmitt bistable.

The input to the drive circuit 13 is taken from the common point of tworesistors R₃₃ and R₃₄ connected in series between the earth rail and oneside of a capacitor C₇. The other side of the capacitor C₇ is connectedto the collector of a pnp transistor P₄, the emitter of which isconnected to the +8v rail and the collector of which is connected by aload resistor R₃₅ to the earth rail. The base of the transistor P₄ isconnected to the common point of two resistors R₃₆, R₃₇ connected inseries between the +8v rail and the collector of an npn transistor N₄,the emitter of which is connected to the ground rail. The base of thetransistor N₄ is connected to the common point of two resistors R₃₈, R₃₉connected in series between the earth rail and one side of a capacitorC₈, the other side of which is connected to the output terminal of theamplifier A₂ . Said one side of the capacitor C₈ is also connected tothe cathode of a diode D₅, The anode of which is connected to the earthrail.

The transistors N₄ and P₄ turn on for a time dependent on the timeconstant of the capacitor C₈ with the resistor R₃₈, R₃₉ (typically about0.7 ms) when the output of the amplifier A₂ goes high. At the same timea short duration positive going pulse is passed to the drive circuit 13.

The capacitor C₈ and resistors R₃₈, R₃₉ provide part of a minimumoff-time circuit which is associated with the Schmitt bistable circuit14. The remainder of this minimum off-time circuit is provided by aresistor R₄₀ and a diode D₆ in series between the collector of thetransistor P₄ and the non-inverting input terminal of the amplifier A₂.These components ensure that, however the input to the Schmitt bistablecircuit behaves immediately following the output of amplifier A₂ goinghigh, the output of amplifier A₂ will not go low again for a presettime, i.e. until transistors P₄ and N₄ switch off when capacitor C₈ ischarged up, because of the additional heavy position feedback to theamplifier A₂.

The input to the drive circuit 12 is derived from the signal at thecollector of the transistor P₄ via a monostable delay circuit based onan operational amplifier A₃. The collector of the transistor P₄ isconnected via a capacitor C₉ and a resistor R₄₁ in series to theinverting input terminal of the amplifier A₃. The non-inverting inputterminal of this amplifier is connected by a bias resistor R₄₂ to the+8v rail and by a positive feedback resistor R₄₃ to the output terminalof amplifier A₃. A diode D₇ has its anode connected to the outputterminal of the amplifier A₃ and its cathode connected by a resistor R₄₄to the inverting input terminal of the amplifier A₃ to provide negativefeedback when the output of amplifier A₃ is high, a capacitor C₁₀connecting the cathode of the diode D₇ to the earth rail.

When transistor P₄ turns on the output of the amplifier A₃ goes lowuntil capacitor C₁₀ has discharged through the resistor R₄₄. The outputof amplifier A₃ then goes high again the capacitor C₉ having meanwhilebecome fully charged and remains high until transistor P₄ turns onagain. The output terminal of amplifier A₃ is connected to the earthrail via a capacitor C₁₁ and two resistors R₄₅, R₄₆ and the input to thedrive circuit 12 is taken from the common point of these resistors.

The armature chopper circuit shown in FIG. 4 includes a main thyristorTH1, a commutating thyristor TH2 and a "ring-round" thyristor TH3, whichare connected to be triggered by the drive circuits 11, 12 and 13respectively. The main thyristor TH1 has its cathode connected via amain fuse 50 to an earth conductor and its anode is connected to one endof the armature winding 51 of the motor. The other end of the armaturewinding 51 is connected via a contact RL3a of the contactor RL3 to ahigh voltage positive supply conductor, the positive and negative supplyconductors being connected by the isolator contacts 48 to the terminalsof a high voltage (e.g. 200-300 volts) battery. A power diode D₈ has itscathode connected to said other end of the armature winding 51 and itsanode connected by an auxiliary fuse 52 to the earth conductor. Thisdiode is operative during braking, when the contact RL3a is open. Afurther power diode D₉ has its anode connected to the anode of thethyristor TH1 and its cathode connected to the supply rail. This furtherdiode is operative to conduct decaying armature current each time themain thyristor TH1 is turned off. A resistor R₅₀ and a capacitor C₁₂ areconnected in series between the anode and cathode of the thyristor TH1.

The commutating thyristor TH2 has its anode connected to the anode ofthe main thyristor TH1 and its cathode connected by a fuse 53 and anindicator 54 to one side of a commutating capacitor C₁₃, the other sideof which is connected to the earth conductor. Said one side of thecapacitor C₁₃ is also connected via a resistor R₅₁ and a diode D₁₀ inseries to the supply rail. The "ring-around" thyristor TH3 has itscathode connected to the earth conductor and its anode connected via aninductor 55 to said one side of the capacitor C₁₃.

When thyristor TH1 is fired current flows through the armature 51 andthe relay contact RL3a (assuming this to be closed). When the armaturecurrent reaches a sufficiently high level for the output of theamplifier A₂ (FIG. 3) to be driven high, the thyristor TH3 is firedimmediately and the thyristor TH2 is fired after the delay mentionedabove. The capacitor C₁₃ is positively charged at this time, chargehaving been maintained if the thyristor TH1 has been conducting for along period by current trickling into the capacitor C₁₃ via the resistorR₅₁. When thyristor TH3 fires, the capacitor C₁₃ commences dischargingthrough the inductor 55, peak current being reached as the capacitor C₁₃becomes completely discharged. Current continues to flow in the inductor55, however, charging capacitor C₁₃ to a peak reverse voltage at whichthyristor TH3 ceases to conduct. The delay set by the delay circuitconstituted by the monostable circuit A₃ is longer than the time takenby this "ring-around" operation. When thyristor TH2 is fired, however,the armature current is diverted in the now reversed charged capacitorC₁₃, allowing thyristor TH1 to turn off. This diverted armature currentcontinues to flow until the capacitor C₁₃ again becomes fully charged inthe original sense whereupon thyristor TH2 turns off and the continuingarmature current (now decaying) flows through the diode D₉. When thearmature current has fallen low enough to cause the output of amplifierA₂ to go low again thyristor TH1 is fired. In this way the armaturecurrent is kept between predetermined limits relative to the demandedarmature current.

FIG. 4 also shows the current signal generator 16 of FIG. 1, whichincludes an operational amplifier A₄ with it inverting and non-invertinginput terminals connected by resistors R₅₂ and R₅₃ to the outputterminals of a Hall plate device 56 energized by two resistors R₅₄, R₅₅connecting it to the +8v and earth rails respectively. A resistor R₅₆connects the non-inverting input terminal of amplifier A₄ to the earthrail and an npn transistor N₅ is connected as an emitter follower to theoutput terminal of the amplifier A₄. A resistor R₅₇ and a diode D₁₁ inseries connect the collector of the transistor N₅ to the +8v rail andthe emitter of this transistor is connected by two resistors R₅₈ and R₅₉in series to the inverting input terminal of the amplifier A₄ so thatthis operates as a difference amplifier. The emitter of the transistorN₅ is connected by a temperature compensation network to the groundrail, such network consisting of two resistors R₆₀, R₆₁ in series and athermistor RT₁ connected across the resistor R₆₀. The output to thedifference amplifier 15 is taken from the junction of the resistors R₆₀,R₆₁. To provide a logic output when there is no difference input to theamplifier A₄ (i.e. when the armature current is zero), a pnp transistorP₅ has its base connected to the collector of the transistor N₅, itsemitter connected to the +8v rail and its collector connected to themotor/brake logic circuit 44 (see FIG. 2 and FIG. 7b).

Turning now to FIG. 5, the field current control circuit includes thestart up circuit 30 based on an npn transistor N₆ which has its emitterconnected to the ground rail and its base connected to the common pointof two resistors R₆₃, R₆₄ across a capacitor C₁₄ connected at one sideto the ground rail and at the other side by a resistor R₆₅ to the outputterminal of the amplifier A₂ (FIG. 3). A capacitor C₁₆ connects thecollector of the transistor N₆ to the ground rail such collector beingalso connected by a resistor R₆₆ to the output terminal of thedifference amplifier 15 (see FIG. 3). The transistor N₆ is normally on,capacitor C₁₅ charging up whenever the output of amplifier A₂ is highand discharging only slowly when the output of amplifier A₂ is low (i.e.when thyristor TH1 is on). Should the output of amplifier A₂ remain lowfor an extended period (indicating that the demanded current cannot beachieved) transistor N₆ turns off, permitting its collector to followthe signal at the output of the difference amplifier 15.

The collector of the transistor N₆ is connected to the anode of a diodeD₁₃, the cathode of which is connected by a resistor R₇₀ to theinverting input terminal of the difference amplifier 29 (FIG. 1). Thenon-inverting input terminal of amplifier 29 is connected by a biasresistor R₇₁ to the +8v rail and a feedback resistor R₇₂ connects theoutput terminal of the amplifier 29 to its inverting input terminal. Theripple rejection circuit shown in FIG. 1 is constituted by a resistorR₁₇₃ and a capacitor C₁₁₇ in series between the cathode of the diode D1D₁₃ and the non-inverting input terminal of the amplifier 29.

Current can flow into the inverting input terminal of the amplifier 29via the diode D₁₃ whenever the signal at the output terminal of theamplifier 15 is high and the transistor N₆ has turned off. Theseconditions occur only during forward motoring. Alternatively current canflow into the inverting input terminal of the amplifier 29 via a diodeD₁₄ from the output terminal of an amplifier A₅ connected as aninverting amplifier producing an output dependent on the differencebetween the signal at the output of the amplifier 15 and a referencevalue (set by a resistor R₇₃ connecting the non-inverting input of theamplifier A₅ to the +8v rail). The amplifier A₅ has its invertingterminal connected by a resistor R₁₇₄ to the terminal A and by aresistor R₁₇₅ to its output terminal. This current flow occurs onlyduring braking. In either event, when the current flowing into theinverting input terminal of amplifier 29 rises above that flowing intothe non-inverting input terminal, the output of the amplifier 29 willfall linearly below a normal high value.

The output terminal of the amplifier 29 is connected by two resistorsR₇₅, R₇₆ to the +8v rail. Three further resistors R₇₇, R₇₈ and R₇₉ inseries connect the junction of the resistors R₇₅, R₇₆ to the groundrail. The junction of the resistors R₇₇, R₇₈ is connected to the cathodeof a diode D₁₅, the anode of which is connected to the output terminalof an operational amplifier A₆. The inverting input of this amplifier A₆is connected to its output terminal by a resistor R₈₀ and to the +8vrail by a resistor R₈₁. The non-inverting input of the amplifier A₆ isconnected by a resistor R₈₂, to the emitter of the transistor N₂, N₃(FIG. 3). Amplifier A₆ acts as a non-inverting amplifier of the voltageon the resistor R₁₃ (FIG. 3) to boost field current at high armaturecurrent demand levels.

The junction of the resistors R₇₈, R₇₉ is connected by a resistor R₈₃ tothe base of an npn transistor N₇ the emitter of which is connected tothe earth rail via a resistor R₈₄ and the collector of which isconnected to the collector of the transistor P₂ (FIG. 3) which is onwhenever a demand for motoring or braking is present.

The transistor N₇ can be turned off either by the signal at a terminal F(see FIG. 7b) going low or an npn transistor N₈ being turned on.Transistor N₈ has its emitter connected to the ground rail, itscollector connected to the base of the transistor N₇ (and to terminal F)and its base connected to the junction of two resistors R₈₅ and R₈₆which are in series between a terminal F' (FIG. 7b) and the ground rail.Terminal F' is also connected to the anode of the diode D₂ (FIG. 3) sothat transistor N₈ can turn on only when the signal at the terminal F'is high and transistor N₁ (FIG. 3) is off (indicating that neither brakenor accelerator pedal is depressed).

The transistor N₇ acts (when transistor P₂ is on) as an emitter followerand its emitter is connected by a resistor R₈₇ to the non-invertinginput terminal of the field current difference amplifier 27, whichterminal is also connected to the +8v rail by a resistor R₈₈. A resistorR₈₉ connects the inverting input terminal of amplifier 27 to the +8vrail and another resistor R₉₀ connects this terminal to the output ofthe field current signal generator 29 which is similar to the armaturecurrent signal generator 16 (shown in detail in FIG. 4). Feedback aroundthe amplifier 27 is provided by a resistor R₉₁ and a capacitor C₁₇ inparallel with each other between the output and inverting inputterminals of the amplifier 27.

The Schmitt trigger bistable circuit 25 of FIG. 1, is constituted by anoperational amplifier A₇ with a resistor R₉₂ connecting the outputterminal of amplifier 27 to the inverting input of amplifier A₇. Thenon-inverting input of amplifier A₇ is connected by a resistor R₉₃ tothe +8v rail and by two resistors R₉₄, R₉₅ in series to the outputterminal of amplifier A₇, a capacitor C₁₈ being connected across theresistor R₉₅.

The output of amplifier 27 rises and falls linearly with the errorbetween the demanded field current and the actual field current. Insteady state conditions R₉₄, R₉₅ provide a small positive feedbackcurrent which establishes hysteresis in the operation of amplifier A₇ sothat the output of amplifier A₇ goes low when the output of amplifier 27rises above one set level and goes high when the output amplifier 27falls below a lower set level. The capacitor C₁₈ introduces additionalpositive feedback for a short period immediately following each changein level of the output of amplifier A₇, thereby inhibiting a furtherchange in level for this period, irrespective of how the output ofamplifier 27 behaves.

The output terminal of amplifier A₇ is connected by a resistor R₉₆ tothe base of an npn transistor N₈ which has its emitter connected to theearth rail and its collector connected by a resistor R₉₇ to the +8vrail. A further npn transistor N₉ has its base connected to thecollector of the transistor N₈, its emitter connected to the earth railand its collector connected by a resistor R₉₈ and the light-emittingdiode of the opto-isolator 26.

Turning now to FIG. 6 it will be seen that the photo-transistor of theopto-isolator 26 has its base connected by a resistor R₁₀₀ to the earthrail and its emitter connected directly to the same rail. The collectorof the photo-transistor is connected by a resistor R₁₀₁ to a +12v railconnected to a tapping on the traction battery and is also connected tothe base of an npn transistor N₁₀ which has its emitter connected to thenegative supply rail and its collector connected by a resistor R₁₀₂ tothe +12v rail. The collector of the transistor N₁₀ is connected to thebase of an npn transistor N₁₁ the emitter of which is connected to theearth rail. An npn transistor N₁₂ has its collector connected to thebase of the transistor N₁₁ and its emitter connected to the earth rail.The base of the transistor N₁₂ is connected to the collector of an npntransistor N₁₃ which has it emitter connected to the earth rail and itscollector connected by a resistor R₁₀₃ to the +12v rail. The base of thetransistor N₁₃ is connected by a resistor R₁₀₄ to the earth rail and bytwo resistors R₁₀₅, R₁₀₆ to the anode of a zener diode ZD₁, the cathodeof which is connected to the +12v rail. The collector of the transistorN₁₂ by a resistor R₁₇₆ is connected to the junction of the resistorsR₁₀₅ and R₁₀₆.

The collector of the transistor N₁₁ is connected to the cathode of adiode D₂₀, the anode of which is connected by a resistor R₁₀₇ to thecathode of a diode D₂₁, the anode of which is connected to the +12vrail. The cathode of the diode D₂₁ is connected by a capacitor C₂₀ tothe earth rail. The cathode of the diode D₂₀ is connected by tworesistors R₁₀₈, R₁₀₉ in series to the +12v rail. The anode of the diodeD₂₀ is connected by two resistors R₁₁₀ and R₁₁₁ in series to the earthrail. An npn transistor N₁₄ has its base connected to the junction ofthe resistors R₁₁₀ and R₁₁₁ and a pnp transistor P₆ has its baseconnected to the junction of the resistors R₁₀₈ and R₁₀₉. Thetransistors N₁₄ and P₆ have their emitters connected respectively to thenegative supply rail and the +12v and their collectors interconnected bya resistor R₁₁₂.

The collector of the transistor P₆ is connected by a resistor R₁₁₃ and acapacitor C₂₁ in parallel to the base of an npn transistor N₁₅ theemitter of which is connected to the base of an npn transistor N₁₆ theemitter of which is connected to the negative supply rail. Thecollectors of the transistors N₁₅ and N₁₆ are connected together and adiode D₂₂ has its cathode connected to the base of the transistor N₁₅and its anode connected to the base of the transistor N₁₆. Thecollectors of the transistors N₁₅, N₁₆ are connected to the normallyclosed contact of a changeover contact set RL2a of the relay RL2 andalso to the normally open contact of a change-over contact set RL2b ofthe relay RL2. The other contacts of these two contact sets areconnected together and via a normally closed contact RL1a of relay RL1to a high voltage supply via a filter F_(R). The field winding 24 isconnected between the common terminals of the contact sets RL2a, RL2b.The collectors of transistors N₁₅, N₁₆ are also connected by a diode D₂₃to the output of the filter F_(R), a further diode D₂₄ connecting thesaid other contacts of the contact sets RL2a, RL2b to the negativesupply rail. In addition, a capacitor C₂₂ connects the collectors of thetransistors N₁₅, N₁₆ to the anode of a diode D₂₅ the cathode of which isconnected to the negative supply rail. A resistor R₁₁₄ is in parallelwith the diode D₂₅. A resistor R₁₇₇ in series with a capacitor C₂₃connects the cathode of diode D₂₃ to the cathode of diode D₂₄, a diodeD₂₆ bridging the resistor R₁₇₇.

Referring now to FIG. 7a, the +12v supply is connected to one side of anignition switch 40. The other side of the switch 40 is connected to thecommon pole of a three way direction selector switch 60 having reverse,neutral and forward contacts. The reverse and forward contacts areconnected to the anode of two diodes D₃₀, D₃₁ which have their cathodesconnected together and, via a resistor R₂₀₀ to the cathode of a zenerdiode ZD₂ the anode of which is connected to the earth rail. A pair ofresistors R₂₀₁ and R₂₀₂ connect the reverse contact of the selectorswitch 60 to the earth rail, the junction of these resistors beingconnected to the base of a npn transistor N₁₇ which has its emitterconnected to the earth rail and its collector connected to a terminalmarked g (see FIG. 7b). The neutral contact is connected to the anode ofa diode D₃₂ the cathode of which is connected via the power relaywinding 40 to the collector of an npn transistor N₁₈. The relay 41 hasnormally open contacts 41a which control the supply of power to all thecontrol circuits. For energising the relay 40 there is a further diodeD₃₃ which has its anode connected to the +12v supply rail controlled bythe contacts 41a and its cathode connected to the cathode of the diodeD₃₂. A freewheel diode D₃₄ is connected across the relay winding 41.

The base of the transistor N₁₈ is connected by a resistor R₂₀₃ to theearth rail and by two resistors R₂₀₄, R₂₀₅ in series to the cathodes oftwo diodes D₃₅ and D₃₆. The anode of the diode D₃₅ is connected to theneutral contact of switch 60, and the anode of the diode D₃₆ isconnected to the collector of a pnp transistor P₇. The emitter of thetransistor P₇ is connected to the +12v and its base is connected to thissame rail by a resistor R₂₀₆ and by two resistors R₂₀₇, R₂₀₈ in seriesto the collector of an npn transistor N₁₉. A capacitor C₃₀ connects thecommon point of resistors R₂₀₄ and R₂₀₅ to the earth rail and acapacitor C₃₁ connects the junction of the resistors R₂₀₇ and R₂₀₈ tothe +12v rail.

A diode D₃₇ has its anode connected to the collector of the transistorP₇ and its cathode connected by two resistors R₂₀₉, R₂₁₀ in series tothe earth rail and by two resistors R₂₁₁, R₂₁₂ in series to the +12vrail. A capacitor C₃₂ is connected between the earth rail and the commonpoint of the resistors R₂₀₉, R₂₁₀ which point is also connected to thecathode of a diode D₃₈ the anode of which is connected to the cathode ofthe zener diode ZD₂. Two resistors R₂₁₃ and R₂₁₄ are connected in seriesacross the capacitor C₃₂ and their junction is connected to the base ofthe transistor N₁₉. A pnp transistor P₈ has its base connected to thecommon point of the two resistors R₂₁₁, R₂₁₂, the emitter of thetransistor P₈ being connected to the +12v rail and its collector beingconnected via a resistor R₂₁₆ to a terminal f (see FIG. 7b).

A further diode D₃₉ has its anode connected to the common point of theresistors R₂₀₉, R₂₁₀ and its cathode connected to the collector of annpn transistor N₂₀ and also connected to the anodes of two diodes D₄₀and D₄₁ the cathodes of which are connected to two interlock functions,one being a switch contact in a charger plug and the other being acontact which is normally held open when the battery voltage is not toolow for satisfactory operation. The emitter of the transistor N₂₀ isconnected to the earth rail and its base is connected to the commonpoint of two resistors R₂₁₇, R₂₁₈ in series between the collector of annpn transistor N₂₁ and the earth rail. The emitter of the transistor N₂₁is connected to the earth rail and its base is connected to the commonpoint of two resistors R₂₁₉ and R₂₂₀ in series between the switch 40 andthe earth rail. The collector of the transistor N₂₁ is connected by aresistor R₂₂₁ to the +5v supply rail. A resistor R₂₂₂ and a capacitorC₃₃ are connected in series between the base of the transistor N₂₀ andthe +5v rail. The collector of the transistor N₂₀ is connected by aresistor R₂₂₃ to the cathode of a diode D₄₂ the anode of which isconnected to a terminal h (see FIG. 7b).

An npn transistor N₂₂ has its emitter connected to the earth rail andits collector connected to the cathode of the zener diode ZD₂. The baseof the transistor N₂₂ is connected by a resistor R₂₂₄ to the earth railand is also connected to the cathode of a diode D₄₄. The anode of thediode D₄₄ is connected by two resistors R₂₂₅ and R₂₂₆ to two terminalsmarked SP>0 (see FIG. 7c) and D>0 (see FIG. 3). A resistor R₂₂₇ and acapacitor C₃₄ in series connect the anode of the diode D₄₄ to the +5vrail. A diode D₄₅ has its anode connected to the anode of the diode D₄₄and its cathode connected to the collector of the transistor N₁₉.

Turning now to FIG. 7b, the terminal h is connected to the base of a pnptransistor P₉ which has its emitter connected to the +5v rail. The baseof the transistor P₉ is connected by a resistor R₂₄₀ to the +5v rail andby two resistors R₂₄₁ and R₂₄₂ in series to the output terminal of anexclusive OR gate G1 (1/4 of a TTL integrated circuit type 7486). Thejunction of resistors R₂₄₁, R₂₄₂ is connected by a capacitor C₃₅ to theearth rail. The anode of a diode D₄₆ is connected to the collector ofthe transistor P₉ and its cathode is connected by three resistors R₂₄₃,R₂₄₄ and R₂₄₅ in series to the rail, a capacitor C₃₆ being connectedbetween the junction of resistors R₂₄₃, R₂₄₄ and the earth rail. Thejunction of the resistors R₂₄₄, R₂₄₅ is connected to the base of an npntransistor N₂₃ which has its emitter connected to the earth rail and itscollector connected to the terminal F (See FIG. 5).

The collector of the transistor P₉ is connected by two resistors R₂₄₆,R₂₄₇ in series to the earth rail, the common point of these resistorsbeing connected to the base of a transistor N₂₄, which has its emitterconnected to the earth rail and its collector connected by a resistorR₂₄₈ to the +5v rail. An npn transistor N₂₅ has its base connected tothe collector of the transistor N₂₄, its emitter connected to the earthrail and its collector connected by the winding of the relay RL1 to the+12v rail, a freewheel diode D₂₇ being connected across this winding.

The collector of the transistor N₂₅ is also connected to the cathode ofa diode D₄₇, the anode of which is connected by three resistors R₂₄₉,R₂₅₀ and R₂₅₁ in series to the +5v rail. A capacitor C₃₇ is connectedbetween the common point of resistors R₂₄₉, R₂₅₀ and the +5v rail andthe base of a pnp transistor P₁₀ is connected to the common point of theresistors R₂₅₀ and R₂₅₁. The emitter of the transistor P₁₀ is connectedto the +5v rail and its collector is connected by two resistors R₂₅₂,R₂₅₃ in series to the earth rail. The base of an npn transistor N₂₆ isconnected to the common point of the resistors R₂₅₂, R₂₅₃ and itsemitter is connected to the earth rail. The collector of the transistorN₂₆ is connected by a resistor R₂₅₄ to the +5v rail and also by acapacitor C.sub. 38 and a resistor R₂₅₅ in series to the base of thetransistor P₁₀. The base of the transistor N₂₆ is connected by tworesistors R₂₅₆ and R₂₅₇ in series to the terminal marked I>0 (see FIG.4) and a capacitor C₃₉ is connected between the junction of theseresistors and the earth rail.

The collector of the transistor P₁₀ is connected by a resistor R₂₆₀ tothe cathode of a diode D₄₈, the anode of which is connected to thecollector of a pnp transistor P₁₁ which has its emitter connected to the+5v rail. A resistor R₂₆₁ connects the base of the transistor P₁₁ to the+5v rail and a resistor R₂₆₂ connects the same base to the outputterminal of another exclusive OR gate G2. The anode of the diode D₄₈ isalso connected to the anode of a diode D₄₉, the cathode of which isconnected by three resistors R₂₆₃, R₂₆₄ and R₂₆₅ in series to the earthrail. A capacitor C₄₀ is connected between the earth rail and thejunction of the resistors R₂₆₃ and R₂₆₄. A diode D₅₀ has its cathodeconnected to the cathode of the diode D₄₉ and its anode connected to theN terminal of the switch 60 (FIG. 7a). A further diode D₅₁ has itscathode connected to the junction of the resistors R₂₆₃ and R₂₆₄ and itsanode connected via a resistor R₂₆₆ to the anode of the diode D₄₆. Thejunction of the resistors R₂₆₄ and R₂₆₅ is connected to the base of annpn transistor N₂₇ which has its emitter connected to the earth rail andits collector connected to the terminal M (see FIG. 3). A pair ofresistors R₂₆₇ and R₂₆₈ are connected in series between the earth railand the junction of the resistor R₂₆₃ and R₂₆₄. An npn transistor N₂₈has its base connected to the junction of these resistors R₂₆₇ and R₂₆₈,its emitter connected to the earth rail and its collector connected tothe terminal BK (see FIG. 3).

Also connected to the cathode of the diode D₄₉ is the cathode of a diodeD₅₂, the anode of which is connected to the collector of a pnptransistor P₁₂. The emitter of the transister P₁₂ is connected to the+5v rail and its base is connected by a resistor R₂₆₉ to the +5v and byresistor R₂₇₀ to the collector of a pnp transistor P₁₃. The emitter ofthe transistor P₁₃ is connected to the +5v rail and its base isconnected by a resistor R₂₇₁ to the +5v rail and by a resistor R₂₇₂ tothe output terminal of a further exclusive OR gate G3. The collector ofthe transistor P₁₃ is connected by a resistor R₂₇₃ to the earth rail andalso by a resistor R₂₇₄ to the terminal F' (see FIG. 5).

An npn transistor N₂₉ has its emitter connected to the earth rail andits collector connected by a resistor R₂₇₅ to the +5v rail and alsoconnected to one input terminal of the gate G2. The base of thetransistor N₂₉ is connected by a resistor R₂₇₆ to the earth rail and bytwo resistors R₂₇₇ and R₂₇₈ in series to the +5v rail. The junction ofthe resistors R₂₇₇ and R₂₇₈ is connected by a contact RL3b, of thecontactor RL3 to the earth rail and also via a resistor R₂₇₉ to theanode of a diode D₅₃ whose cathode is connected to the base of thetransistor N₂₇. The collector of the transistor N₂₉ is connected to theanode of a diode D₅₄ the cathode of which is connected by a resistorR₂₈₀ to the base of the transistor N₂₈.

The relay winding RL2 is connected between the +12v rail and thecollector of an npn transistor N₃₀, the emitter of which is connected tothe earth rail, a diode D₂₈ being connected across this winding. Thebase of the transistor N₃₀ is connected by a resistor R₂₈₁ to the earthrail and by a resistor R₂₈₂ to the collector of a pnp transistor P₁₄,the emitter of which is connected to +5v rail. The base of thetransistor P₁₄ is connected by a resistor R₂₈₃ to the +5v rail and by aresistor R₂₈₄ to the Q output terminal of a bistable latch circuit L₁(which may be 1/4 of a TTL integrated circuit type 7475). The DATA inputof the latch L₁ is connected to the output terminal of another exclusiveOR gate G4, and its CLOCK input terminal is connected to the collectorof the transistor N₂₆. The DATA input terminal of the latch L₁ and its Qoutput terminal are connected to the two input terminals of the gate G1.

The gate G4 has one input terminal connected by a resistor R₂₈₅ to the+5v rail and also connected to the collector of an npn transistor N₃₁,the emitter of which is connected to the earth rail. The base of thetransistor N₃₁ is connected by a resistor R₂₈₆ to the earth rail, by aresistor R₂₈₇ to the terminal MTR/BK (see FIG. 3) and by a resistor R₂₈₈to the cathode of a diode D₅₅, the anode of which is connected to theearth rail. The cathode of the diode D₅₅ is also connected by acapacitor C₄₁ to the cathode of a diode D₅₆, which cathode is connectedto the ground rail by a resistor R₂₈₉. A capacitor C₄₂ connects thecathode of the diode D₅₆ to the cathode of a diode D₅₇ which is alsoconnected to the earth rail by two resistors R₂₉₀, and R₂₉₁ in series,the anode of the diode D₅₇ being connected to the earth rail. An npntransistor N₃₂ has its base connected to the junction of the resistorsR₂₉₀, R₂₉₁ and also to the resistor R₂₁₆ (see FIG. 7a) via terminal f.The emitter of the transistor N₃₂ is connected to the ground rail andits collector is connected to the cathodes of two diodes D₅₈ and D₅₉,the anode of the diode D₅₈ being connected to the terminal F' and theanode of the diode D₅₉ being connected via two resistors R₂₉₂, R₂₉₃ inseries to the earth rail. The anode of the diode D₅₉ is also connectedby a resistor R₂₉₄ to the anode of the diode D₅₆, which is alsoconnected to the collector of a pnp transistor P₁₅. The emitter of thetransistor P₁₅ is connected to the +5v rail, and its base is connectedto this rail by a resistor R₂₉₅. A resistor R₂₉₆ connects the base ofthe transistor P₁₅ to the Q output terminal of a bistable latch circuitL₂ which has its CLOCK input connected to the collector of thetransistor N₂₆ and its DATA input connected to the collector of thetransistor N₃₁. The Q output terminal of the latch L₂ is also connectedto an input terminal of the gate G2, via terminal m.

The other input terminal of the gate G4 is connected to the Q outputterminal of a third bistable latch circuit L₃ which has its DATA inputterminal connected by a resistor R₂₉₇ to the +5v rail and also connectedto the collector of the transistor N₁₇ (see FIG. 7a). The CLOCK inputterminal of the latch L₃ is connected by a resistor R₂₉₈ to the +5v railand is also connected to a terminal n (see FIG. 7c).

The junction of the resistors R₂₉₂ and R₂₉₃ is connected to the base ofan npn transistor N₃₃ which has its emitter connected to the earth railand its collector connected by a relay winding RL4 to the +12v rail, therelay RL4 having a normally open contact controlling the contact RL3.The relay winding RL4 is bridged by a freewheel diode D₂₉ and acapacitor C₂₄ bridging its normally open contact.

Turning finally to FIG. 7c, a bridge rectifier BR1 has its inputterminals connected to opposite ends of the motor armature winding. Thenegative output terminal of the bridge rectifier is connected by a rail70 to one end of a transformer primary T₁. The positive output terminalof the bridge rectifier is connected by a resistor R₃₀₀ to the cathodeof a zener diode ZD₃, the anode of which is connected to the rail 70. Acapacitor C₅₀ is connected across the zener diode ZD₃. The unijunctiontransistor U₁, has its base 1 connected to the rail 70, its base 2connected by a resistor R₃₀₁ to the cathode of the zener diode ZD₃ andits emitter connected by a resistor R₃₀₂ to the cathode of the zenerdiode ZD₃. A capacitor C₅₁ is connected between the emitter of theunijunction transistor U₁ and the other end of the primary winding T₁, adiode D₆₀ being connected across the winding T₁.

The secondary winding T₂ of the transformer is connected at one end tothe +5v rail. Its other end is connected by a resistor R₃₀₃ to the anodeof a diode D₆₁ and the cathode of a diode D₆₂, the cathode of the diodeD₆₁ being connected to the +5v rail. The anode of the diode D₆₂ isconnected by a capacitor C₅₂ to the +5v rail and by a resistor R₃₀₄ tothe base of a pnp transistor P₁₆. A resistor R₃₀₅ connects the base ofthe transistor P₁₆ to the +5 l v rail and the emitter of this transistoris connected to the same rail. The collector of the transistor P₁₆ isconnected to the terminal marked SP>O and is also connected by aresistor R₃₀₆ to the base of an npn transistor N₃₄, the emitter of whichis connected to the earth rail. A resistor R₃₀₇ connects the base of thetransistor N₃₄ to the earth rail and the collector of the transistor N₃₄is connected by a resistor R₃₀₈ to the base of the transistor P₁₆ toprovide latching action. The collector of the transistor N₃₄ isconnected to the terminal n and is also connected by a capacitor C₅₃ anda resistor R₃₀₉ in series to the base of a pnp transistor P₁₇. Theemitter of the transistor P₁₇ is connected to the +5 v rail and itscollector is connected by a resistor R₃₁₀ to the base of the transistorN₃₄. The base of the transistor P₁₇ is connected by a resistor R₃₁₁ tothe +5 v rail and by two resistors R₃₁₂, R₃₁₃ in series to the anode ofa diode D₆₃, the cathode of which is connected (via a terminal p) to theoutput terminal of the gate G3. A capacitor C₅₄ connects the junctionbetween the resistors R₃₁₂, R₃₁₃ to the earth rail.

Having, thus, described the various components shown in FIGS. 7a to 7cand the connections between them, the operation of the logic circuitswill now be explained in detail.

At start up, when the ignition switch 40 is closed and the switch 60 isin neutral position the transistor N₁₈ can turn on, being supplied withbase current through the diode D₃₅ and the resistors R₂₀₄ and R₂₀₅.Current can then flow through the diode D₃₂ and the relay coil 41. Thecontacts of this replay provide power to the circuits and also relaylatching current via the diode D₃₃. Initially, the transistor P₈ isturned on via resistors R₂₁₂, R₂₀₉, R₂₁₀ so that its collector is high.The capacitor C₄₀ charges up via the diode D₅₀ and the resistor R₂₆₃from the neutral contact of the switch 60. Capacitor C₄₀ holds on bothtransistors N₂₇, and N₂₈ which act to clamp the motoring and brakingdemand signals to zero.

When the switch 60 is moved to, say, the forward position, thetransistor N₁₉ receives base current via diodes D₃₀ and D₃₈ andresistors R₂₀₀ and R₂₁₃. Transistor N₁₉ conducts and causes transistorP₇ to turn on via the resistors R₂₀₈ and R₂₁₇. This is a latchingarrangement and transistor N₁₉ is held on via diode D₃₇, and resistorsR₂₀₉ and R₂₁₃. The transistor P₇ also keeps the transistor N₁₈ on viathe diode D₃₆ and the resistors R₂₀₄ and R₂₀₅, so that the relay 41remains energised. The transistor P₈, however, loses its supply of basecurrent because the transistor P₇ collector has gone high and P₈therefore turns off, the function of which will be explainedhereinafter. Also transistors N₂₇ and N₂₈ turn off after a delay whilstthe capacitor C₄₀ discharges, since base current via the neutral switchposition has been lost, so that an armature current demand can now bemade.

There are certain restrictions and conditions governing the selection offorward and reverse drive for safety reasons. Firstly, the transistorN₂₁ is turned on by a +12 v supply from the vehicle "ignition" switch 40through the resistor R₂₁₉. This keeps the junction of the resistors R₂₁₇and R₂₂₁ normally low (and transistor N₂₀ therefore off) except whenfirst powering up the circuit when the +5 v supply rises rapidly andtransistor N₂₀ is turned on via the capacitor C₃₃ and resistor R₂₂₂whilst capacitor C₃₃ is charging up. Transistor N₂₀ turns on thetransistor P₉ through the resistor R₂₂₃ and the diode D₄₂ during thisperiod (approximately 25 mS) for reasons explained hereinafter. At thesame time another capacitor/resistor combination C₃₄ and R₂₂₇ hold thetransistor N₂₂ on for about 150 mS. Transistor N₂₂ prevents transistorN₁₉ from turning on during this time because of the connection of thecollector of transistor N₂₂ to the junction of resistor R₂₀₀ and diodeD₃₈. These delays ensure correct starting up of the circuit whenswitching on and immediately selecting forward or reverse drive. Thereare also four interlock signals which prevent forward or reverse beingselected, these being the charger plug interlock and auxiliary batterylow cut-out which when low (Ov) hold the cathode of diode D₃₉ low viadiode D₄₀ or D₄₁ and prevent transistor N₁₉ from turning on. The othertwo interlocks are provided by the D>O and SP>O signals, which, whenhigh, turn transistor N₂₂ on and also prevent transistor N₁₉ turning on,so that if transistor N₁₉ is on these two signals can have no effectbecause transistor N₂₂ is held off by the diode D₄₅ and the transistorN₁₉.

When the "ignition" switch 40 is opened transistor N₂₁ loses its basedrive and turns off, so that transistor N₂₀ turns on and removes thebase current from the transistor N₁₉ via diode D₃₉. After a delay ofabout 200 mS, while capacitor C₃₁ discharges, the transistor P₇ turnsoff and transistor P₈ turns on. This makes the collector of transistorP₈ go high which is used, as explained hereinafter to make the contactorRL3 open. The transistor P₉ has previously turned on and clamped all thedemand signals by turning on transistors N₂₃, N₂₇ and N₂₈ so that nocurrent will be flowing in the armature and field windings. After afurther delay of about 150 mS whilst capacitor C₃₀ is discharging thetransistor N₁₈ turns off causing the relay 41 to drop out and removepower from the entire circuit. The vehicle can also be shut down bytaking either of the interlock diodes D₄₀, D₄₁, to Ov, the same sequencethan taking place.

When the vehicle is actually being driven forward (or is in the forwarddrive connection made and at rest) and reverse drive is required, switch60 is operated to select reverse drive. In this reverse position basecurrent for the transistor N₁₇ is provided by resistor R₂₀₁ andtransistor N₁₇ therefore holds the DATA input of the latch L₃ low. Thelatches can only change state when the signals at their CLOCK inputs arehigh and in the case of the latch L₃, this can only occur if the vehicleis not moving, movement being detected by sensing the voltage across themotor armature with a small field current flowing. If a voltage ispresent the motor must be rotating, the bridge rectifier BR1 detectingsuch voltage (the polarity of which will depend on whether motoring orbraking mode is in operation). The output of the bridge rectifier BR1 isdropped by the resistor R₃₀₀ and the zener diode ZD₃ to +15 v. CapacitorC₅₀ provides smoothing during chopping when the armature voltage varies.The voltage across the capacitor C₅₀ drives an oscillator based on theunijunction transistor U₁, and the pulses generated by the oscillatorare transferred to the remainder of the circuit through the pulsetransformer T₁, T₂. The diodes D₆₁, D₆₂, the resistor R₃₀₃ and thecapacitor C₅₂ form a diode pump circuit which acts to turn on thetransistor P₁₆ via resistor R₃₀₄. The transistor P₁₆ turns on transistorN₃₄ and also holds the CLOCK input of the latch L₃ low to prevent changeover of the latch.

When a change from forward to reverse is demanded the following sequenceoccurs:- The demanded change is detected and a small field current isapplied to the motor. After a delay, if no armature voltage has beendetected, the change over is allowed. Normally the output of gate G3 islow because both of its inputs are the same. Transistor P₁₃ is on as istransistor P₁₇ (via diode D₆₃ and resistors R₃₁₂, R₃₁₃) so thattransistor N₃₄ is on preventing the change over of latch L3. When thedirection change is demanded the DATA input to latch L₃ goes high, sothat one input of gate G3 also goes high as does the output of gate G3.Transistor P₁₃ turns off and transistor P₁₂ turns on thereby clampingthe armature demand via transistors N₂₇ and N₂₈. Transistor P₁₃ turningoff removes the supply of the base of transistor N₈ (FIG. 5) viaresistors R₂₇₄ and R₈₅ (FIG. 5). This releases the base of transistor N₇(FIG. 5) so that a minimum field current demand is applied to the fieldchopper. A back emf is therefore generated if the motor is rotating.During this time the diode D₆₃ has become reverse biased and capacitorC₅₄ discharges through the resistor R₃₁₂ into the base of transistor P₁₇and after a delay of about 100 mS transistor P₁₇ turns off andtransistor N₃₄ also turns off, provided that transistor P₁₆ is not on.Turning off of transistors P₁₇ and N₃₄ is speeded up by the capacitorC₅₃ and resistor R₃₀₉ providing positive feedback, thereby ensuring afast edge to the signal at the CLOCK input of the latch L₃ which isdesirable for interference-free operation. The latch L₃ can now changeto the required state and its Q output is compared by gate G4 with themotor/brake demand signal from the collector of the transistor N₃₁. Theoutput of the gate G4 goes to latch L₁ whose Q output drives the fieldreversing relay RL2 via resistor R₂₈₄ transistor P₁₄, resistor R₂₈₂ andtransistor N₃₀. This circuit arrangement gives the required fieldcurrent directions for forward and reverse, motoring and braking asshown, in the following table:-

    ______________________________________                                        Operat-                                Field  Re-                             ing    L3                              Direc- lay                             mode   DATA    L3.sup.--Q                                                                           G4 in G4 out                                                                              L1.sup.--Q                                                                         tion   RL2                             ______________________________________                                        Forward                                                                              High    Low    Low   Low   High Normal Off                             Motoring                                                                      Forward                                                                              High    Low    High  High  Low  Reverse                                                                              On                              Braking                                                                       Reverse                                                                              Low     High   Low   High  Low  Reverse                                                                              On                              Motoring                                                                      Reverse                                                                              Low     High   High  Low   High Normal Off                             Braking                                                                       ______________________________________                                    

The relay RL1 is arranged to be operated for a sufficient time when achangeover is demanded to ensure reduction of the field current to zero.Thus the reversing relay RL2 is never required to break any currentwhich could damage its contacts. The change-over sequence is asfollows:-

The Q output of latch L₃ changes as described above and the changepasses through the gate G4 to the latch L₁ DATA input. However, latch L₁cannot change immediately because transistor N₂₆ is held on bytransistor P₁₀, and transistor N₂₅. The demanded change is detected bygate G₁ whose output goes low and turns on transistor P₉ which turns ontransistor N₂₄. The demand clamping transistors N₂₃, N₂₇ and N₂₈ arealso turned on by the gate G1 output, thereby ensuring that there are nofield or armature current demands. The transistor N₂₄ clamps the base oftransistor N₂₅ to Ov and therefore de-energises the relay RL1 so thatits contacts open and the field current decays rapidly to zero. After adelay somewhat longer than that required to reduce the field current tozero (about 75 mS), the capacitor C37 discharges and transistor P₁₀turns off. Thus transistor N₂₆ loses its base current from resistorsR₂₅₂ and also turns off, so that the latch L₁ CLOCK input goes high andlatch L₁ can change to its new state. Gate G1 then changes back to itsnormal high output state, the relay RL1 is re-energised and the clampingtransistors are turned off after short delays produced by capacitor C₃₆with resistor R₂₄₄ and capacitor C₄₀ with resistors R₂₆₄ and R₂₆₇ toallow the relays to attain their correct conditions. The vehicle is thendrivable in the required direction. The same sequence of events occursin the opposite direction of change over, except that the latches andgates end up in the required state as shown in the table above.

Field forcing also occurs on switch on by the turning on of thetransistor P₉ via transistor N₂₀ to ensure that no field or armaturecurrent demand is made before the relays have settled in their normaloperating positions. At shut down a similar sequence occurs when theignition switch is opened or one of the interlock diodes D₄₀, D₄₁ istaken low.

The change over from motoring to braking is very similar to theforward-reverse change over. In motoring the MTR/BK selector (i.e. theoutput of amplifier A₁ FIG. 3) is high so that transistor N₃₁ is on andthe Q output of latch L₂ is low. When the MTR/BK signal goes low, theDATA input of latch L₂ and one input of gate G4 change state, latch L₁therefore changes when transistor N₂₆ turns off as described above, alsolatch 12 changes at the same time. Thus transistors P₁₅ and N₃₃ turnoff, gate G2 detects the change (because of the delay in contactor RL3operation). Transistor N₂₉ turns off and causes N₂₇ to turn on viaresistors R₂₇₈, R₂₇₉ and diode D₅₃ (in braking) or transistor N₂₈ toturn on via resistor R₂₇₅ and R₂₈₀ and diode D₅₄ in motoring. Thisensures that the braking demand is clamped in motoring and vice versa.While the Q output signal of latch L₁ and the the output of the gate G2is low and the transistor P holds the transistors N₂₇ and N₂₈ on. Thesedamand clamps are released after the contactor RL3 has reached its newposition, that is transistor P₁₅ has turned off and removed drive totransistor N₃₃ and the relay RL4, so that the brake contact RL3 is inits de-energised state which is the normally open (braking) position.There are two further safety interlocks to protect the contactor:-namely on I>O signal from the current transducer to transistor N₂₆ toprevent the latch changing if an armature current is flowing andtransistor P₁₁ conducts during change over to hold transistor N₂₆ on sothat the latch cannot change back to its original state until brakinghas been obtained and transistor P₁₁ turns off. These interlocks ensurethat there is no possibility of breaking a fault current.

Because of residual magnetism in the motor it is desirable to ensurethat the field current is reversed before the contactor RL3 is closedinto the motoring position to prevent large uncontrolled currents beinggenerating in the armature 51 and recirculating diode D₉. The I>Ointerlock then prevents changing back into braking unless the motoringfield is applied to reset the magnetic circuit in the motor. Byresetting the field before closing the contactor, the problem isavoided. During braking, when change over to motoring is demanded theMTR/BK signal goes high so that the collector of transistor N₃₁ goeslow, the field reversal process takes place exactly as described abovethrough gate G4 and latch L₁ after a delay for field forcing. The latchL₂ also changes at the same time and transistor P₁₅ turns on so that itscollector goes high. This positive-going edge is transmitted throughdiode D₅₆, capacitor C₄₂ and resistor R₂₉₀ to turn on transistor N₃₂which removes the base drive to transistor N₃₃ through diode D₅₉ so thattransistor N₃₃ cannot turn on. The relay RL4 therefore staysde-energised and the contactor RL3 stays in the open (braking) conditionuntil capacitor C₄₂ charges up and transistor N₃₂ turns off. During thisdelay, the Q output of the latch L₂ and the contact RL3b are out ofphase so that gate G2 output is low and the armature demand is clampedby transistor P₁₁ supplying base current to transistor N₂₇ and N₂₈. Thefield reversing replay has already changed and a reset signal isprovided by holding off the field clamp via diode D₅₈ and transistor N₃₂so that transistor N₈ turns off and allows a small field demand to bemade at the emitter of transistor N₇ (the demand is for a current ofapproximately 2 amps). Because the transistor N₃₂ serves the dual roleof holding the contactor in braking and allowing a field current to flowin the correct (motoring) direction, the field resetting pulse mustalways occur at the right instant, i.e. after the field has reversed,but before the braking contact RL3 closes. When the transistor N₃₂ goesoff, the brake contactor RL3 closes, the output of gate G2 goes high andthe armature demand clamps are released after capacitor C₄₀ hasdischarged. The vehicle is then in the motoring mode with the magneticcircuit set in the right direction so that the recirculating diode D₉ isreverse biased. To ensure that the field pulse always occurs correctly,the duration of the motoring signal at transistor N₃₁ should be longerthan the time required to change from braking to motoring. This isachieved by another capacitor-resistor network C₄₁, R₂₈₈ connected tothe base of transistor N₃₁ to hold that transistor on when the collectorof transistor P₁₅ goes high until the capacitor C₄₁ is charged. Thisdelay is made slightly longer than the contactor hold-off delay producedby capacitor C₄₂ and resistor R₂₉₀, so that rapid changes from motoringto braking and back again do not interfere with the operation of thefield reset circuits. The diodes D₅₅ and D₅₇ and the resistor R₂₈₉ alsocontribute by allowing capacitors C₄₁ and C₄₂ to discharge quickly afterselecting braking ready for the next change into motoring.

An extra function of these components occurs on start up. In neutraltransistor P₈ is on and holds transistor N₃₂ on via the resistor R₂₁₆.This holds the contactor in its braking condition and produces a smallfield current. If the vehicle is moving transistor P₁₆ will turn ontransistor N₂₂ and prevent either forward or reverse being selected.

I claim:
 1. A control system for a d.c. motor having an armature windingand a field winding, comprising control means for varying connections ofsaid armature winding and/or field winding so as to enable the motor tooperate in a plurality of different modes, armature current regulatingmeans connected to vary the armature current, field current regulatingmeans connected to vary the field current and speed interlock means forpreventing an operation of said control means demanding a mode changethat changes said connections from at least one motor operation enablingmode to at least one other such mode while the motor is running, thespeed interlock means being sensitive to voltage across said armaturewinding, wherein the improvement comprises:interlock control means whichoperate to control current in the field winding when said mode change isdemanded, said interlock control means controlling said field currentregulating means so as sequentially to cause the field winding currentto be reduced to substantially zero and then to supply a controlledcurrent pulse to said field winding, the speed interlock means acting todetect the armature voltage during said controlled current pulse.
 2. Acontrol system as claimed in claim 1 wherein said speed interlock meansincludes a rectifier connected to the armature winding and a meanssensitive to the d.c. output of the rectifier.
 3. A control system asclaimed in claim 2 in which said means sensitive to the d.c. output ofthe recitifier includes and oscillator, an isolating transformer havingits primary winding connected to the oscillator and a detector circuitconnected to the secondary winding of the isolating transformer.
 4. Acontrol system as claimed in any one of claims 1, 2 or 3 wherein saidinterlock control means operates to supply said field wind with a fixedlength current pulse of predetermined magnitude as said controlledcurrent pulse.